High precision, low phase noise synthesizer with vector modulator

ABSTRACT

An apparatus for generating a modulated signal which includes a modulator operative for receiving a first signal and input data signals, and modulating the first signal in accordance with the input data signals so as to produce a modulated reference signal; a first frequency divider coupled to the modulator output, operative to reduce the frequency of the modulated reference signal by a predetermined factor; a signal generator operative to produce a second signal; a first mixer having a first input coupled to an output of the first frequency divider and a second input coupled to the output of the signal generator. The first mixer operates to frequency translate the modulated reference signal by an amount equal to the frequency of the second signal. In addition, the signal generator of the present invention contains a direct digital synthesizer coupled to a second phase lock loop which operates to up-convert the output signal of the DDS to the microwave region. The feedback network of the second phase lock loop utilizes frequency translation to down-convert the output signal in order to close the loop.

BACKGROUND OF THE INVENTION

[0001] Various systems, such as cellular communication systems, utilizehigh frequency modulated signals to transmit data between differentlocations. The use of the high frequency modulated signal allows thedata to be transmitted via a wireless link and therefore avoids thecostly expense of running cable between the locations.

[0002] Typically, in such systems, an intermediate frequency signal isgenerated by a local oscillator. This first intermediate frequencysignal is then up-converted (i.e., frequency multiplied) so as toproduce a carrier signal having a frequency suitable for transmissionvia a wireless link, for example, Ku or C band. The carrier signal isthen modulated in accordance with the data to be transmitted.Alternatively, the intermediate frequency signal can be modulated priorto up-converting the signal to the carrier frequency.

[0003] The use of the intermediate frequency signal provides numerousadvantages. For example, in many communication systems the location ofthe antenna for transmitting and receiving modulated carrier signals isdistant from the components generating and processing the data. Byutilizing intermediate frequency signals to transmit data betweencomponents contained in the same location, the need for expensive cable,which would be necessary to transmit high frequency carrier signals withacceptable loss and leakage levels, is eliminated. Accordingly, wirelesscommunication systems typically up-convert the intermediate frequencysignal to the carrier frequency as one of the last steps in generatingthe modulated carrier signal.

[0004] A problem remains in that prior art systems typically utilizeconventional phase lock loops (“PLL”) to up-convert the intermediatefrequency signal. The use of such PLLs, while raising the frequency asrequired, also results in a substantial increase in the phase noiseassociated with the signal. The increase in phase noise degrades thespectral purity (i.e., increases the noise floor) of the signal, whichis undesirable.

[0005] Another problem with prior art systems pertains to the frequencydeviation characteristics of the modulator utilized to superimpose thedata to be transmitted on the intermediate frequency signal prior to theup-conversion process. Frequency deviation relates to the amount theoutput of the modulator deviates from the desired frequency. In otherwords, the frequency error or frequency variance generated by themodulator As such, it is desirable to minimize the frequency deviationas much as possible.

[0006] In prior art systems which modulate the intermediate frequencysignal and then up-convert the signal to the carrier frequency, thefrequency deviation or error is also multiplied by the same factor asthe frequency, which can be on the order of 100 or greater.Specifically, the frequency error increases in accordance with ratiobetween the carrier and the intermediate frequency. As a result, thefrequency of the modulated carrier signal is susceptible to undesirablevariations.

[0007] Accordingly, there exists a need for an apparatus for generatinga modulated carrier signal suitable for transmission via a wireless linkwhich negates the foregoing problems.

SUMMARY OF THE INVENTION

[0008] The present invention relates to an apparatus for producing a lowphase noise, high precision, modulated signal. Specifically, theinvention comprises a novel synthesizer/ modulator design which allowsfor the modulation and frequency multiplication of an intermediatefrequency signal without a corresponding increase in the frequencydeviation or phase noise of the signal.

[0009] Accordingly, the present invention relates to an apparatus forgenerating a modulated signal comprising a modulator operative forreceiving a first signal and input data signals, and modulating thefirst signal in accordance with the input data signals so as to producea modulated reference signal; a first frequency divider coupled to themodulator output, operative to reduce the frequency of the modulatedreference signal by a predetermined factor; a signal generator operativeto produce a second signal; a first mixer having a first input coupledto an output of the first frequency divider and a second input coupledto the output of the signal generator. The first mixer operates tofrequency translate the modulated reference signal by an amount equal tothe frequency of the second signal.

[0010] In addition, the signal generator of the present inventioncomprises a direct digital synthesizer (“DDS”) coupled to a phase lockloop which operates to up-convert the output signal of the DDS to themicrowave region. The feedback network of the second phase lock looputilizes frequency translation to down-convert the signal in order toclose the loop.

[0011] As described in detail below, the apparatus of the presentinvention provides important advantages. For example, by down-convertingthe modulated reference signal output by the modulator, the frequencydeviation of the modulated reference signal is also reduced by the samefactor. Accordingly, inexpensive modulators operating in the microwaveregion can be utilized. Without such down-conversion, upon completingthe up-conversion process, the frequency deviation of the modulatedreference signal would exceed the frequency deviation rating of themodulator.

[0012] Furthermore, the down-conversion of the modulated referencesignal also provides the added benefit that any error in the I/Q balanceof the input data signals to the modulator is also reduced by the samefactor.

[0013] In addition, by utilizing frequency translation in the feedbackloop of the phase lock loop contained in the signal generator, thepresent invention increases the frequency of the output signal of theDDS without increasing the phase noise or spurious frequency componentsof the signal.

[0014] The invention itself, together with further objects and attendantadvantages, will best be understood by reference to the followingdetailed description, taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1 is a block diagram of the synthesizer/modulator of thepresent invention.

[0016]FIG. 2 is a schematic diagram of one embodiment of the apparatusof FIG. 1.

DETAILED DESCRIPTION OF THE DRAWINGS

[0017] Referring to FIG. 1, the synthesizer/modulator of the presentinvention comprises a first phase lock loop 2 having an input port forreceiving a reference signal 3 from a local oscillator, or equivalentthereof, and an output port which is coupled to an input port of amodulator 4. The first PLL 2 functions to frequency multiply thereference signal 3 to a predetermined frequency.

[0018] The modulator 4, for example, a quadrature or phase modulator,receives modulation data 5 to be superimposed on the up-convertedreference signal generated by the first PLL 2. The modulated referencesignal generated by the modulator 4 exhibits full scale frequencydeviation in accordance with the frequency deviation rating of themodulator 4.

[0019] It is noted that the modulation data 5 represents the informationto be transmitted. The type of modulation data 5 input to the modulator4 varies in accordance with the modulation scheme being utilized. Forexample, if the modulation scheme utilized is quadrature phase shiftkeying “QPSK,” the modulator 4 would be a QPSK modulator and themodulation data 5 input into the modulator 4 could be a serial datastream.

[0020] The modulated reference signal is coupled to a frequency divider6, which down-converts the modulated reference signal by a predeterminedfactor M. However, along with the reduction in frequency, the frequencydivider 6 reduces the frequency deviation of the modulated referencesignal by a factor of M. The output of the frequency divider 6 iscoupled to one input of a mixer 8.

[0021] As explained in greater detail below, the other input to themixer 8 is coupled to the output of a high resolution tunable signalgenerator 10 comprising a DDS 22 and a second PLL 26 which utilizesfrequency translation in the feedback loop (neither the DDS or the PLLare shown in FIG. 1). The tunable signal generator 10 is desirable so asto allow for various techniques, such as, frequency division multipleaccess (“FDMA”) which is utilized in cellular systems.

[0022] The output of the mixer 8 is coupled to a bandpass filter 12 soas to select either the upper or lower sideband of the resultant signal.As a result, the signal present at the output of the bandpass filter 12exhibits low phase noise and a frequency deviation which is less thanfrequency deviation of the modulator 4.

[0023] Further, the first PLL 2 provides a reference signal 25 which iscoupled to the DDS 22 of the signal generator 10 via a frequency/divider24. As explained below, the reference signal 25 functions as a stablelocal oscillator as required by the operation of the DDS 22.

[0024] The first PLL 2 also provides an input 27 to a mixer 36 containedin the feedback loop of the second PLL 26. As explained below, the mixer36 functions to frequency translate the output signal of the second PLL26 to a lower frequency in order to close the feedback loop of thesecond PLL 26. It is this frequency translation which prevents anincrease in the phase noise of the signal generated by the second PLL26.

[0025] Advantageously, the present invention allows for the use ofinexpensive modulators 4, for example, the ATT W2010 I/Q modulator. Asexplained above, the frequency deviation of the modulated signal isreduced by frequency dividing the modulated signal prior toup-converting the signal to the carrier frequency. If the factor bywhich the modulated signal is reduced is greater than the factor bywhich the modulated signal is subsequently increased to produce thecarrier signal, the frequency deviation of the modulated carrier signalwill remain less than the full scale deviation of the modulator 4.

[0026] Accordingly, the design of the synthesizer/modulator of thepresent invention utilizes two feedback loops to generate the low phasenoise, high precision, modulated signal. The first loop comprises thehigh resolution tunable signal generator 10, which utilizes the DDS 22to precisely set the frequency of the signal to be transmitted, and asecond PLL 26 utilizing frequency translation, which raises thefrequency of the signal generated by the DDS 22 without raising thephase noise or the spurious frequency components of the signal output bythe DDS 22.

[0027] The second loop comprises the first PLL 2, the phase modulator 4and the frequency divider 6, and functions to produce a modulated signalhaving a frequency deviation which is less than the frequency deviationof the phase modulator 4.

[0028] The operation of the present invention is explained in furtherdetail in conjunction with the operation of a satellite communicationsystem comprising a plurality of remote ground terminals whichcommunicate with a central hub station via a satellite. Typically, theremote ground terminals comprise a small aperture directional antennafor receiving and transmitting signals to the satellite, an indoor unitfor generating a modulated data signal and for demodulating incomingsignals and an outdoor unit mounted proximate the antenna for convertingthe modulated data signal into a modulated carrier signal. The presentinvention would be utilized to generate the modulated data signal insuch a system.

[0029] Referring to FIG. 2, the first PLL 2 is a standard phase lockloop comprising a phase detector 14 having one input for receiving a10.24 Mhz reference signal, which is generated, for example, by a localoscillator, a low pass filter 16 coupled to the output of the phasedetector 14, a voltage controlled oscillator (“VCO”) 18 coupled to theoutput of the low pass filter 16, a first frequency divider 20 coupledto the output of the VCO 18, and a second frequency divider 21 coupledto the output of the first frequency divider 20. The output of thesecond frequency divider 21 is coupled to a second input of the phasedetector 14 so as to complete the feedback network of the first PLL 2.

[0030] As shown in FIG. 2, the first frequency divider 20 divides theoutput of the VCO 18 by a factor of 4 to produce a 215.04 Mhz referencesignal, and the second frequency divider 21 divides this signal by afactor of 21 so as to produce a reference signal having a frequency of10.24 Mhz.

[0031] As stated above, the output of the first PLL 2, which is a 860.16Mhz reference signal taken at the output of the VCO 18, is coupled tothe input of the modulator 4. The modulator 4 receives modulation data 5and produces a phase modulated signal having a constant amplitude.Acceptable modulation schemes include minimum shift keying (“MSK”)modulation. The phase modulated signal output by the modulator 4 alsoexhibits full scale frequency deviation (i.e., the maximum frequencydeviation as rated by the modulator specification).

[0032] The output of the modulator 4 is coupled to the frequency divider6 which reduces the frequency of the modulated signal and the frequencydeviation by a factor of 16 to produce a modulated reference signalhaving a frequency of 53.76 Mhz, The output of the frequency divider 6is coupled to one input of a mixer 8. The other input to the mixer 8 isprovided by the output of the high resolution tunable signal generator10.

[0033] The high resolution tunable signal generator comprises a DDS 22,for example, the HSP45102 produced by Harris Corp., having a first inputfor receiving a variable frequency signal 23 (i.e., tuning frequency)utilized to tune the output of the DDS 22 to the desired frequency, anda second input for receiving a reference signal 25. The reference signal25 is generated by coupling the output of the first frequency divider 20of the first PLL 2 to a frequency divider 24 which divides the signal bya factor of 8. The output of the frequency divider 24, which is a 26.88Mhz reference signal, is coupled to the second input of the DDS 22. Theoutput of the DDS 22, which is coupled to the input of the second PLL26, is tunable in accordance with the variable frequency signal 23 andin the present embodiment is approximately 6 Mhz.

[0034] The second PLL 26 comprises a phase detector 28 having one inputfor receiving the output signal from the DDS 22, a low pass filter 30coupled to the output of the phase detector 28, a VCO 32 coupled to theoutput of the low pass filter 30, a frequency divider 34 coupled to theoutput of the VCO 32, a mixer 36 having one input port coupled to theoutput of the frequency divider 34, and a bandpass filter 38 coupled tothe output of the mixer 36. The output of the bandpass filter 38 iscoupled to a second input of the phase detector 28 so as to complete thesecond PLL 26. The second input port of the mixer 36 is coupled to theoutput of the first frequency divider 20 of the first PLL 2.

[0035] The operation of the high resolution tunable signal generator 10is as follows. As stated, in the present embodiment, the output of theDDS 22 is tuned so as to generate a reference frequency of 6 Mhz, TheVCO 32 generates a signal having a nominal frequency of 837 Mhz which isdivided by a factor of 4 by the frequency divider 34. The resulting 209Mhz signal is then coupled to the mixer 36, which in conjunction withthe 215.04 Mhz signal input from the first PLL 2, frequency translatesthe 209 Mhz signal to a nominal 6 Mhz signal. This 6 Mhz signal is thencompared to the DDS 22 output signal in the phase detector 28 to closethe loop.

[0036] By utilizing the frequency translation scheme in the feedbackloop, a precisely tunable frequency reference is established thatexhibits phase noise characteristics significantly less than that of asynthesizer utilizing a conventional phase lock loop. Furthermore, theuse of the frequency translation scheme minimizes the increase in thespurious frequency components generated by the DDS 22.

[0037] The output of the high resolution tunable signal generator 10 iscoupled to the other input of mixer 8, which functions to produce anominal 890 Mhz modulated intermediate frequency signal having minimalphase noise and {fraction (1/16)} the frequency deviation produced bythe modulator 4.

[0038] The modulated intermediate frequency signal output by the mixer 8is coupled to an additional frequency divider 40 via bandpass filter 1which further reduces the frequency of the signal and the frequencydeviation by a factor of 8. The resulting signal having a nominalfrequency of 111 Mhz is then coupled to an outdoor unit locatedproximate an antenna. The outdoor unit up-converts the modulated signalto the desired carrier frequency utilizing direct multipliers, such asgallium arsenide FETs. The modulated carrier signal is then coupled toan antenna and transmitted.

[0039] The apparatus of the present invention provides numerousadvantages. For example, by down-converting the modulated referencesignal output by the modulator, the frequency deviation of the modulatedreference signal is also reduced by the same factor. As such,inexpensive modulators operating in the microwave region can beutilized.

[0040] Another advantage obtained by the down-conversion of themodulated reference signal is that any error in the I/Q balance of theinput data signals to the modulator is also reduced by the same factor.

[0041] Yet another advantage is that the present invention increases thefrequency of the output signal of the DDS without increasing the phasenoise or spurious frequency components of the DDS signal. As statedabove, this is accomplished by utilizing frequency translation in thefeedback loop of the phase lock loop contained in the signal generator.

[0042] Numerous variations of the foregoing invention are also possible.For example, the operational frequencies described above relate to thegeneration of a modulated signal having a nominal frequency of 111 Mhz.the foregoing invention can be utilized to generate modulated signalshaving any desired frequency.

[0043] Of course, it should be understood that a wide range of otherchanges and modifications can be made to the preferred embodimentdescribed above. It is therefore intended that the foregoing detaileddescription be regarded as illustrative rather than limiting and that itbe understood that it is the following claims including all equivalents,which are intended to define the scope of the invention.

What is claimed is:
 1. An apparatus for generating a modulated signalcomprising: a modulator operative for receiving a first signal and inputdata signals, and modulating said first signal in accordance with saidinput data signals so as to produce a modulated reference signal; afirst frequency divider coupled to an output of said modulator, saidfrequency divider operative to reduce the frequency of said modulatedreference signal by a predetermined factor; a signal generator operativeto produce a second signal; and a first mixer having a first and secondinput, said first input coupled to an output of said first frequencydivider and said second input coupled to an output of said signalgenerator, said first mixer operative to frequency translate saidmodulated reference signal by an amount equal to the frequency of saidsecond signal.
 2. An apparatus for generating a modulated signalaccording to claim 1, further comprising: a local oscillator forgenerating an rf signal; and a first phase lock loop for multiplying thefrequency of said rf signal by a predetermined amount so as to generatesaid first signal, said first phase lock loop having an input forreceiving said rf signal, and a feedback network comprising a second andthird frequency divider coupled in series.
 3. An apparatus forgenerating a modulated signal according to claim 1, wherein saidmodulator modulates the phase of said first signal in accordance withsaid input data signals such that said modulated reference signalcomprises an envelope of constant amplitude.
 4. An apparatus forgenerating a modulated signal according to claim 2, wherein said signalgenerator comprises: a direct digital synthesizer for generating atunable signal, said synthesizer having a first and second input, saidfirst input operative to receive a tuning frequency signal, said secondinput operative to receive a first reference signal; and a second phaselock loop for multiplying the frequency of said tunable signal by apredetermined amount so as to generate said second signal.
 5. Anapparatus for generating a modulated signal according to claim 4,wherein said second phase lock loop comprises a feedback networkincluding a second mixer having a first and second input, said firstinput coupled to an output of said second phase lock loop and saidsecond input coupled to a second reference signal, said second mixeroperative to frequency translate said second signal by an amount equalto the frequency said second reference signal.
 6. An apparatus forgenerating a modulated signal according to claim 5, wherein said secondinput of said second mixer is coupled to an output of said secondfrequency divider of said first phase lock loop.
 7. An apparatus forgenerating a modulated signal according to claim 4, wherein said secondinput of said direct digital synthesizer is coupled to an output of saidsecond frequency divider of said first phase lock loop.
 8. An apparatusfor generating a modulated signal according to claim 1, wherein saidmodulated signal comprises a frequency in the microwave region.
 9. Amethod for generating a modulated signal comprising: generating a firstsignal and input data signals; modulating said first signal inaccordance with said input data signals so as to produce a modulatedreference signal; frequency dividing said modulated reference signal soas to reduce the frequency of said modulated reference signal by apredetermined factor; generating a second signal by means of a signalgenerator; and frequency translating said modulated reference signal byan amount equal to the frequency of said second signal.
 10. A method forgenerating a modulated signal according to claim 9, further comprising:generating an rf signal by means of a local oscillator; and multiplyingthe frequency of said rf signal by a predetermined amount by a firstphase lock loop so as to generate said first signal, said first phaselock loop having an input for receiving said rf signal, and a feedbacknetwork comprising a frequency divider.
 11. A method for generating amodulated signal according to claim 9, wherein said modulated referencesignal comprises an envelope of constant amplitude.
 12. A method forgenerating a modulated signal according to claim 10, wherein said signalgenerator comprises: a direct digital synthesizer for generating atunable signal, said synthesizer having a first and second input, saidfirst input operative to receive a tuning frequency signal, said secondinput operative to receive a first reference signal; and a second phaselock loop for multiplying the frequency of said tunable signal by apredetermined amount so as to generate said second signal.
 13. A methodfor generating a modulated signal according to claim 12, wherein saidsecond phase lock loop comprises a feedback network including a mixerhaving a first and second input, said first input coupled to an outputof said second phase lock loop and said second input is coupled to asecond reference signal, said second mixer operative to frequencytranslate said second signal by an amount equal to the frequency saidsecond reference signal.
 14. A method for generating a modulated signalaccording to claim 13, wherein said second input of said second mixer iscoupled to an output of said frequency divider of said first phase lockloop.
 15. A method for generating a modulated signal according to claim12, wherein said second input of said direct digital synthesizer iscoupled to an output of said frequency divider of said first phase lockloop.
 16. 16. A method for generating a modulated signal according toclaim 9, wherein said modulated signal comprises a frequency in themicrowave region.